The present invention relates to cuffed endotracheal and tracheostomy tubes (both referred to generally as tracheal tubes) or other types of cuffed tubes which encounter absorptive gases. More particularly, it relates to a tracheal tube having a cuff resistant to absorption of nitrous oxide anesthetic gas.
There are many types of cuffed tubes and catheters for use as medical devices. These include tracheal tubes, Foley catheters, Dennis tubes, rectal catheters, etc. However, tracheal tubes encounter conditions and demand characteristics which are unique. One of the most crucial distinctions is the fact that tracheal tube cuffs, unlike other types of medical tubes, are subjected to anesthetic gases.
Tracheal tubes are used to provide an air-way through the trachea into the lungs of a patient during respirator therapy or general anesthesia. During anesthesia, such tubes allow nitrous oxide and other gases to be administered to the patient in controlled amounts with the tube remaining in the desired location and with the trachea sealed off by the inflated cuff.
The inflatable cuff should provide both airway seal during positive-pressure ventilation and protection from aspiration without causing significant trauma to the trachea. A serious practical dilemma exists because lateral wall pressure adequate to maintain tracheal seal may decrease or eliminate capillary flow in the lamina propria, and cuff-to-trachea pressure that permits capillary flow may also permit gas leak and/or aspiration. In regard to the latter, studies have been done to determine the minimum intra-cuff pressure needed to prevent tracheal aspiration of dye in anesthestized patients whose tracheas are intubated. See, for example, Bernhard et al, "Adjustment of Intracuff Pressure to Prevent Aspiration," Anesthesiology, Vol. 50, p. 363+ (April 1979).
Thus, the recommended procedure has been to carefully adjust the cuff pressure to a point where an adequate tracheal seal is maintained without overinflating the cuff and, thereby, damaging the walls of the trachea.
It is also usually recommended that the newer high volume-low pressure cuffs be used to further aid in preventing tracheal wall damage. See, "Tracheal Tubes", Health Devices, Vol. 7, p. 75+ (Jan. 1978). That article looked at the three general types of tracheal tube cuffs. These are (1) low volume-high pressure cuffs, (2) intermediate cuffs which will tend to seal like high volume-low pressure types if used in relatively narrow tracheas, but which require extra pressure in relatively wide tracheas, and (3) high volume-low pressure cuffs. A thin (around 3-15 mil) soft polyvinyl chloride was used to produce most of the high volume-low pressure cuffs tested in that article.
The Health Devices article goes on to warn, however, that since higher volume cuffs are generally thinner, they are more vulnerable to inward diffusion of nitrous oxide. Thus, the increased use of low pressure cuffs has brought attention to this recognized problem.
A study reported in Stanley, "Nitrous Oxide and Pressure and Volumes of High and Low-pressure Endotracheal-tube Cuffs in Intubated Patients," Anesthesiology, Vol. 42, p. 637+ (May 1975) indicates that cuff overexpansion during anesthesia may be an important cause of tracheal or laryngeal trauma and postoperative sore throat in patients whose tracheas have been intubated. This overexpansion was attributed to diffusion of nitrous oxide into the cuff and slow diffusion of nitrogen out.
A more recent study demonstrates that gas volumes in most cuffs increase 1.7 to 7 ml after 30 minutes of exposture to 100 percent nitrous oxide. This is reported in Bernhard et al, "Physical Characteristics of and Rates of Nitrous Oxide Diffusion into Tracheal Tube Cuffs," Anesthesiology, Vol. 48, p. 413+ (June 1978).
In view of this result, an additional procedure is recommended. As stated in the 1978 Anesthesiology article:
It is difficult to know how porous for nitrous oxide a cuff is just by looking at it. One should, therefore, assume nitrous oxide diffusion if the cuff was inflated with air, and deflation to a "safe" intracuff pressure should be carried out approximately every 30 minutes.
It would clearly be better to avoid the need for such adjustments. Accordingly, a number of proposals have been made which would do this. These can be conveniently divided into two categories: (1) use of cuff pressure regulating devices and (2) use of filled cuffs which have less tendency to absorb nitrous oxide gases. An example of use of valves as inflation control devices in preventing undue cuff pressures is found in Shan U.S. Pat. No. 4,116,201 and Wallace U.S. Pat. Nos. 4,018,231 and 3,901,246. Examples of filled cuffs are found in U.S. Pat. Nos. 3,640,282 (sponge rubber filled cuff); 4,022,217 (liquid saline solution filled cuff); and 3,971,385 (silicone gel filled cuff).
The difficulty with such devices is that they are complex and expensive and were not specifically designed to overcome the problem of nitrous oxide diffusion in the first place. Accordingly, the need exists for a simple, inexpensive and yet effective means to prevent undue pressure rises in tracheal tube cuffs and other tube cuffs which are exposed to anesthetic gases.